## Thursday, March 24, 2011

### Square Footed, Part III

In the past 2 days we have used elementary math skills to calculate the footprint and hat size of a house. Now we are going to learn how much attic space needs to be ventilated (we could say the house's cranial capacity).

Many houses have ventilated attics - outside air circulates freely above the ceiling and below the roof. This reduces build-up of heat and moisture that might damage the house. When you live in moderate coastal or desert areas like San Diego, venting the house is very important.

Here you can see a couple of the attic vents at my house. On this side of the house, we  ventilate the part of the roof that overhangs the exterior walls. I think we have 12 soffit vents to let cool air in, and several large gable vents that let hot air out.

A general building code rule-of-thumb is to use 1 square foot of vent for every 300 square feet of attic space. Let's take a look at our sample house again.

Here the roof outline is shown in light brown. It's roughly 36 squares, or 3600 square feet. If the room represented the attic space, we would need 3600 ÷ 300 = 12 square feet of attic ventilation area (screened vents along the edges and gables, or in the top of the roof itself).

Here is a more detailed look at the house with the rooms shown. The brown areas are porches, and don't have any attic space above them. The garage is open all the way to the roof. The bedroom, living room and entry have cathedral ceilings, and we don't need to ventilate anything that hangs out over the exterior walls of this house.

Using the dimensions on the first drawing, we can calculate that the bedroom is 280 square feet, living 330, porches 500, garage 400, entry 300 and overhang 300. Adding those gives us about 2100 square feet. Subtract that from the total of 3600 and we have a remainder of roughly 1500 square feet of attic space. Divide by 300 to discover we need only 5 square feet of ventilation area. That's less than half of what we assumed before when we used the roof dimensions.

NOTE: Remember that I'm not a contractor or builder, but a creator of problems for elementary math curriculum. Please don't build your house based on the sample drawings or the problems in this blog!